Ribonucleases H from bacteria and retroviruses have been examined for enzymatic activity when either the amino or carboxyl termini were altered. Addition of amino acids at the amino terntinus of Salmonella typhimurium RNase H had little effect on the specific activity of the protein as did a small deletion of the carboxyl terminus. In contrast, removal of the carboxyl one third of the RNase H portion of the AKRMuLV reverse transcriptase RNase H dramatically decreased the RNase H activity without any significant alteration of the polymerase activity. Substitution of seleno- methionine for methionine in E. coli. RNase H does not seem to alter the activity. The mRNA from the RRP1 gene of yeast has three unusual properties: 1) there is a long (for yeast) 3'-untranslated region 2) the level of mRNA decreases as the cell density increases and 3) there is an overlap of sequence of the 3'-terminus of the RRP1 mRNA with the 5'-terminus of a more abundant 0.6 kilobase mRNA. The relationship of these unusual characteristics to the function of the RRP1 protein remain unknown but suggest that the RRP1 protein may be important in regulating the amount of pre-ribosomal RNA converted to mature rRNA.